An output frequency of a frequency synthesizer employing an YIG is generally controlled by regulating a magnetic field applied to the YIG. A YIG oscillator is structured to include a main coil and a frequency-modulation (FM) coil. The main coil is configured in a structure that coils are wounded on an iron core, generating a high magnetic field. The FM coil is formed in an air-cored structure, generating a smaller magnetic field relative to the main coil. By controlling the magnetic fields generated from the main and FM coils, it is able to regulate an output frequency of the frequency synthesizer with the YIG oscillator.
FIG. 1 is a diagram of a general YIG oscillator driving device 100. Referring to FIG. 1, the general YIG oscillator driving device 100 is constructed by including a microprocessor 110, a frequency divider 120, a reference frequency generator 130, a phase detector 140, a loop filter 150, a digital-to-analogue converter (DAC) 160, a voltage-to-current converter (VIC) 170, a resistor 180, and an YIG oscillator 190.
The microprocessor 110 generates digital information correspondent to an output frequency to be output. The microprocessor 110 controls the frequency divider 120, the reference frequency generator 130, and the DAC 160 in response to the digital information.
The frequency divider 120 operates to divide an output frequency of the YIG oscillator 190 under the control of the microprocessor 110. The divided output frequency is transferred to the phase detector 140. The reference frequency generator 130 operates to generate a reference frequency under the control of the microprocessor 110. The reference frequency is supplied into the phase detector 140.
The phase detector 140 receives a frequency, which is divided from the output frequency of the YIG oscillator 190, from the frequency divider 120, and receives the reference frequency from the reference frequency generator 130. The phase detector 140 compares the divided frequency to the reference frequency and then outputs their frequency gap in the form of direct current (DC).
The loop filter 150 operates to smooth the DC output of the phase detector 140 and outputs the smoothed DC output. The loop filter 150 may be made up with a low pass filter (LPF). An output of the loop filter 150 is led to the YIG oscillator 190 by way of the resistor 180. The resistor 180 is provided to prevent an output voltage of the loop filter 150 from being lower.
The DAC 160 receives digital information from the microprocessor 110 and converts the digital information into an analogue signal. The VIC 170 converts an output (analogue signal) of the DAC 160 into a current signal. An output (current signal) of the VIC 170 is led to the YIG oscillator 190.
The YIG oscillator 190 is connected to the loop filter 150 through the resistor 180 and also connected to the VIC 170. The YIG oscillator 190 includes a main coil 192, an FM coil 194, and a frequency generator 196. The main coil 192 of the YIG oscillator 190 is supplied with current from the VIC 170 and generates a magnetic field corresponding thereto. The FM coil of the YIG oscillator 190 is supplied with current from the loop filter 150 through the resistor 180 and generates a magnetic field corresponding thereto. The frequency generator 196 of the YIG oscillator 190 generates an output frequency in response to the magnetic fields generated by the main and FM coils 192 and 194.
Responding to the digital information created by the microprocessor 110, the DAC 160 and the VIC 170 provides a constant current to the main coil 192 of the YIG oscillator 190. The main coil 192 generates the magnetic field in response to the constant current. The frequency divider 120, the reference frequency generator 130, the phase detector 140, the loop filter 150, the resistor 180, and the YIG oscillator 190 constitute a phase-locked loop (PLL). This PLL functions to control the current provided into the FM coil 194, regulating the output frequency of the YIG oscillator 190.
The output frequency of the YIG oscillator 190 is determined by responding to the magnetic fields generated from the main and FM coils 192 and 194. The magnetic field of the main coil 192 is larger than that of the FM coil 194. In other words, the output frequency of the YIG oscillator 190 is more affected from the main coil 192 than the FM coil 194 in magnetic field. Exemplarily, the output frequency of the YIG oscillator 190 may be affected from the main coil 192 in magnetic field of 90%, while from the FM coil 194 in magnetic field of 10%.
The output frequency of the YIG oscillator is variable in accordance with environmental conditions. In particular, if peripheral temperature changes, the output frequency of the YIG oscillator 190 may be sensitive to temperature. The main coil 192 of the YIG oscillator 190 is supplied with a large amount of the constant current (e.g., over hundreds mA). Namely, if temperature of the main coil 192 increases due to the current supplied into the main coil 192, the magnetic field is variable in magnetic field. The main coil 192 acts to charge 90% of the factors determining the output frequency of the YIG oscillator 190. In other words, if the magnetic field of the main coil 192 is changed, the output frequency of the YIG oscillator 190 is also variable.
The YIG oscillator driving device 100 as shown in FIG. 1 first regulates its output frequency on a target frequency by controlling the magnetic field of the main coil 192 with reference to the digital information that is predetermined in the microprocessor 110. Then, the output frequency is set to the target frequency by controlling the magnetic field of the FM coil 194 through the PLL. Eventually, if the output frequency of the YIG oscillator 190 is changed out of the permissible range by the FM coil 194 due to variation of environmental conditions, it is not always to lock the output frequency on the target frequency just by controlling the magnetic field of the FM coil 194.
FIG. 2 is a diagram showing another type of the YIG oscillator driving device for stabilizing an output frequency of the YIG oscillator. Reference numerals as like those used in the YIG oscillator driving device 100 of FIG. 1 denote the elements similar to those of the YIG oscillator driving device shown in FIG. 1. Referring to FIG. 2, an output voltage of a DAC 260 is added to an output voltage of the PLL through adders 272 and 274, and the sum of the output voltages are applied to a main coil 292. As the PLL controls the main coil 292 minutely, it extends a range of tracking the output frequency against environmental conditions and hence improves the stability of the YIG oscillator.
However, even with the YIG oscillator driving device 200 shown in FIG. 2, there is a limit to the range of tracking the output frequency of the YIG oscillator 290 against environmental conditions because the main coil 292 is supplied with a current also with reference to digital information that is predetermined by the microprocessor 210.